Top panel: Na I D absorption feature in our highest S/N spectra. In the +271 d spectrum, the individual narrow components of an absorption doublet at the host redshift can be seen. The resolving powers of the two spectra are listed. Middle panel: He Iλ5876 and Na I D emission feature for SN 2021irp and SN 2017hcc at a similar epoch, normalised at the emission line peak. Velocity is with respect to the midpoint of the Na doublet. Both SNe display an absorption feature superimposed in the emission profile. The narrower absorption at ∼ − 6000 km s⁻¹ is the MW Na absorption. Bottom panel: Absorption line in the spectrum of SN 2021irp associated with Ca H. It has a very similar width and blueshift to the Na D feature.